This are the scripts I used to use years ago:
671,Silver,Silver Coin,2,,,10,,,,,0xFFFFFFFF,7,2,,,,,,{ if((Zeny+(50000000))<(1000000001)){set Zeny,Zeny+(50000000);}else{dispbottom "Your zeny would go above 1,000,000,000 if you use this!"; getitem 671,1; } },{},{}
673,Gold,Gold Coin,2,,,10,,,,,0xFFFFFFFF,7,2,,,,,,{ if((Zeny+(100000000))<(1000000001)){set Zeny,Zeny+(100000000);}else{dispbottom "Your zeny would go above 1,000,000,000 if you use this!"; getitem 673,1; } },{},{}
675,Platinum,Platinum Coin,2,,,10,,,,,0xFFFFFFFF,7,2,,,,,,{ if((Zeny+(500000000))<(1000000001)){set Zeny,Zeny+(500000000);}else{dispbottom "Your zeny would go above 1,000,000,000 if you use this!"; getitem 675,1; } },{},{}
677,Bronze,Bronze Coin,2,,,10,,,,,0xFFFFFFFF,7,2,,,,,,{ if((Zeny+(10000000))<(1000000001)){set Zeny,Zeny+(10000000);}else{dispbottom "Your zeny would go above 1,000,000,000 if you use this!"; getitem 677,1; } },{},{}
You might have to change the values to your needs.